When a high voltage is applied between two metal pieces separated by a small gap, the voltage difference between the metal pieces stresses the insulating medium situated in the gap. That insulating medium could be a gas, such as the air, or a liquid that is dielectric in character. Should the gap be small, the voltage could be large enough to ionize and electrically break-down the insulating medium, producing an electric arc, or, as variously termed, a spark that jumps across the gap between those opposed surface locations on the metal pieces which, as viewed microscopically, are closest to one another. The spark conducts electrical current from the source of high voltage through the metal.
The electric spark releases energy in various forms, including visible light, ion and electron acceleration, acoustic energy and heat. That released energy, principally believed to be the heat, has an erosive effect on the metals, referred to as electroerosion. Minute amounts of metal sputters from the surface of both metal pieces. Although noting the physical effect of sparks on the metal, the theoretical physics underlying that erosion is not fully understood by the present applicant and, as becomes apparent, is not necessary to the understanding of the present invention.
Because of its erosive effect, electrical sparks have heretofore been used to cut metals, form complex three dimensional cavities within metals and otherwise shape metal surfaces in three dimensions, a process referred to as electric discharge machining (EDM). Apparatus to perform EDM usually employs a shaped metal piece, called the electrode, to electroerode the other metal piece, usually larger in size, the workpiece, which is to be cut and/or shaped. To date EDM has become quite sophisticated. Three types of EDM apparatus are presently used for discharge machining: sinking EDM, numerically controlled three dimensional generic electrode EDM, and wire EDM.
Sinking is the main type of EDM used for making mold parts and other work, where the cut is not composed of straight lines going through the workpiece. In the sinking process, one or more electrodes are prepared to mimic a "positive" image to the "negative" image of the cavity desired. With both the electrode and workpiece immersed in a dielectric "flushing" fluid, the electrode is gradually pushed into the mold material as sparks erode the mold. Debris is flushed out from the spark gap by circulating the flushing fluid.
With sinking, electrode wear is usually an important factor: sharp edges and fine details need sequential roughing and finishing operations with re-surfacing of the electrode in between. However, flushing is the single most important limitation of sinking process speed and accuracy. Since the gap between electrode and workpiece is typically less than 0.005 of an inch and gap areas often exceed one or more square inches, special provisions are often needed to help dielectric (machining) fluid flow through the gap. Regular, frequent stops in cutting action and withdrawal of the electrode from the workpiece to increase the gap and permit a higher rate of fluid flow, are normal functions of existing EDM sinking operations. Material removal rate is very slow, usually less than one cubic inch (1 in.sup.3) per hour); electrode preparation is expensive and operator involvement is often required.
Where the size of the workpiece permits, conventional machining, being much faster than sinking EDM, is often used initially to rough out the final shape as a shortcut to speed up the metal removal process. After such pre-machining, the workpiece is often heat treated to its final hardness, and then finished with sinking type EDM.
As an advantage, an aspect of the present invention permits removal of large volumes of metal from a workpiece, expeditiously, much more quickly than by the sinking process, effectively scooping out a chunk of metal at a time from the workpiece, and is particularly useful in working hard or brittle materials, such as carbides, or hard-to-machine work pieces, such as fragile or thin walled shapes and the like.
The numerically controlled three-dimensional EDM process employs a generic electrode, such as a small tube. The term "generic electrode", as accepted in the art, refers to an axial non-formed tool electrode of a simple machining surface contour, which may be cylindrical, triangular or square in cross section and which is generally dissimilar or independent of the three-dimensional shape of a final cavity or contour to be machined in a workpiece. Such a "generic electrode" is distinguished from the formed tool electrode used in the sinking process that is a mirror image or a scaled-down or scaled-up image of the three-dimensional cavity or contour desired in the workpiece.
In the three-dimensional EDM process with at least one generic tool electrode having a machining surface contour at an end portion thereof, the tool electrode is axially juxtaposed with a workpiece to position the machining surface contour in spaced juxtaposition therewith across an EDM gap and the gap is supplied with a machining liquid, a dielectric fluid. A succession of electrical discharges are produced across the EDM gap to electroerosively remove stock from a localized portion of the workpiece. To advance the process, the tool electrode is displaced relative to workpiece the along a three-dimensional path, typically under numerical control, a digital computer, while maintaining the EDM gap, whereby the desired cavity or contour, dissimilar to the generic electrode and basically determined by the path of the three dimensional feed displacement effected between the tool electrode and the workpiece, is carved out in the workpiece. Examples of such numerically controlled EDM apparatus is presented in patents to Inoue, U.S. Pat. No. 4,543,460, granted Sep. 24, 1985, and U.S. Pat. No. 4,606,007, granted Aug. 12, 1986 and in Shimizu, U.S. Pat. No. 4,608,476 granted Aug. 26, 1986.
The advantages of the generic electrode EDM process over the conventional "sinking" EDM process, which makes it essential to use one or more similar formed electrodes of mirror images of a desired cavity or contour, are increasingly recognized in the art. It is very difficult to prepare a formed tool electrode of a precise mirror image of a desired cavity or contour required in the sinking EDM process. In addition, several such electrodes of slightly varying sizes are often required to allow repetition of the process in different modes ranging from roughing to finishing. Because of such factors, the sinking EDM process for machining a three-dimensional cavity or contour has been very costly and laborious.
By contrast, in the three dimensional EDM process a simple tool electrode in the form of a cylinder of small cross-section or the like, or more than one such simple electrode varying in size can simply be employed to machine a large and/or intricate cavity or contour. The cavity or contour is machined in the workpiece by displacing the generic electrode and the workpiece relative to each other, under numerical control or sequence copying control, along a prescribed three-dimensional path programmed in the NC digital computer which determines the final cavity or contour desired in the workpiece. Since the generic electrode is allowed to move generally in an open space to advance machining, the process does not present a problem as the depth into the workpiece increases as is encountered by sinking EDM.
Even with that advantage, the three dimensional numerical control EDM process has not been accepted as a replacement for the sinking process in the manufacture of molds. Because of the large volumes of stock that must be removed from the workpiece to construct the mold, existing three dimensional EDM apparatus is, perhaps, simply too slow. An unavoidable ancillary effect of electric discharge machining is that the generic electrode also erodes, and must be replaced frequently, which contributes to the delay in completing machining. As an advantage, the present invention employs three-dimensional EDM, but does not require frequent electrode replacement. Ideally, EDM apparatus constructed in accordance with the present invention may operate virtually unattended.
Generic electrodes used in 3D EDM apparatus are usually rigid, unlike wire, which is malleable and can be readily bent by hand. One partial exception is a specialized micro-precision EDM equipment developed recently, such as the Panasonic brand model MG-ED720W EDM apparatus, a miniaturized machine. That miniaturized EDM machine uses a short length of very fine wire as the generic electrode, which is held to an associated mandrel by a ceramic cylindrical member, referred to by the manufacturer as a wire guide. Being so fine, the wire is fragile and if not carefully handled it can of course be bent. Preparation of that electrode wire and the subsequent EDM process is found to be very time consuming. Moreover, that equipment is not a full 3D machine, but may be characterized as a two and one-half dimension one. It handles a two dimensional shape that is "extruded" along the third axis. The lack of compensating mechanism for wear on the micro-fine electrode limits that process to through-holes or slots.
Further the micro-fine wire electrode in that Panasonic apparatus is elongated, and extends, cantilever fashion, a great distance in terms of the wire's radius, from its support. It must be of such a length to machine a passage through the workpiece, as example, and hence must be long enough to extend through the workpiece to the latter's bottom surface. Movement of flushing liquid or high velocity gas used during machining around the fine wire electrode may cause the wire to vibrate or deflect, which compromises machining accuracy. Additionally, when the discharge current passes through that relatively long fine wire electrode during EDM machining, electromagnetic forces generated by that current can also deflect the wire, changing its position. That also compromises machining accuracy. As an advantage, the present invention provides minimum exposure of the electrode wire, only a short length, and thereby avoids the problem of magnetic deflection or flushing fluid deflection to maintain the highest degree of machining accuracy.
Creating a texture or artwork on the inside of a cylinder or cone of a mold using generic electrodes simply has not been practical. And, at present, apart from the present invention, no EDM machine or device known to applicant is able to fully form surfaces on metal work pieces using a the tip of a fine wire. As an advantage, one aspect of the present invention is of a generic electrode that is in the form of a wire, and includes rather fine wires. Using the tip of a fine wire as the electrode, the invention possesses the capability of shaping metal surfaces, including applying texture and artwork thereto, as well as forming through-holes and slots.
Putting aside the foregoing Panasonic miniaturized EDM unit, what is customarily referred to in the industry as a "wire EDM" is an EDM apparatus in which the electrode is wire, such as copper wire typically ranging from 0.006 inches to 0.016 inches in diameter, and that wire is fed from a supply spool located to one side of the work surface onto a take-up spool located on the other side. The wire is under tension and essentially streams past the workpiece, "cutting" the workpiece using only the straight section of the wire that extends between the two spools. The foregoing operation closely mimics sawing with a round jigsaw blade.
The force deflecting the wire in that wire EDM apparatus is mainly generated from the cutting current's electro-magnetic effects. Therefore excellent accuracy is obtained if the current is not excessive for the wire diameter used. The effect of electrode (wire) wear on cutting accuracy is kept negligible by feeding a large quantity wire past the cutting surface at a relatively high velocity. It may be noted that a patent to Inoue, U.S. Pat. No. 4,629,855 granted Dec. 16, 1986, suggests use of a flat metal tape or ribbon instead of a cylindrical wire for the foregoing type of EDM system.
Like sawing, the actual volume of metal cut out and removed in the foregoing way is usually many times greater than the volume of metal actually electro-erosively converted into minuscule particles. Therefore, wire EDM is able to produce finished parts relatively fast, even though the slender wire only allows very little "cutting" current to erode the metal.
If the cut does not start on the exterior of the workpiece, it is necessary to first form a starting hole in the workpiece to allow the wire to be threaded through that hole. Fabrication of a starting hole in a workpiece is a sinking operation in which pre-fabricated tubular electrodes are used. For through-holes a cylindrical tube can be used; for blind holes a web is necessary through the inner diameter. The tube is usually rotated during electro-erosion to produce through-holes, which is discretionary, but for blind holes the tube must be rotated and the web removes the standing cylinder of material that an ordinary tube would leave behind. Like wire EDM, the simple tube appears to remove more material than what is eroded. Rotating webbed tubes however requires that all material removed be converted into debris. Modern wire EDM machines function with minimum operator attention, a benefit to that process.
As an advantage the present invention eliminates the necessity for both the sinking EDM and present generic electrode EDM's. It also offers an alternative to, but does not necessarily replace the foregoing wire type EDM. One aspect of the present invention provides a numerically controlled three dimensional EDM apparatus, enhanced with robotic tool substitution apparatus, that is able to efficiently accomplish in a single system the machining accomplished by any of those prior EDM apparatus types.
A common and more specific issue to each of the foregoing EDM systems, and, particularly for wire EDM systems is electrode wear. Not only does the EDM apparatus electro-erode the workpiece, it incidentally erodes the electrode as well, though to a lesser degree. Electrode wear has two adverse effects. If, left uncompensated, electrode wear increases the length of the spark gap. Even with computer controlled compensation to maintain the proper length to the gap, ultimately, the electrode wears down to the point at which the EDM apparatus must be shut down and the worn electrode replaced.
For proper machining operation, the length of the spark gap is important and, desirably, should be maintained essentially constant. Technical data on electrode wear rates is known and, typically, that information is stored in the 3D systems numerical control digital computers. That information is typically used by the computer to make adjustments to the tool assembly to workpiece displacement as a function of the time duration of machining, essentially moving the tool assembly. That supports the electrode, a little more close to the workpiece as a function of time. Thus as the electrode shortens, the computer control automatically corrects for the increasing height of the EDM gap by moving the tool drive assembly and, hence, the electrode, closer to the workpiece.
The foregoing gap adjustment to account for electrode wear is based on prediction, and not actual measurement. An example of such is described in the numerically controlled EDM process presented in the patent to Diot et al., U.S. Pat. No. 5,354,961, granted Oct. 11, 1994. In this predictive technique a metal "wear" calculation is based on preexisting technological data and parameters concerning the tool shape, material, and duration of use; and the numerical control apparatus provides appropriate signals to the three dimensional control unit a signal continuously compensating for such electrode wear. As one appreciates, predictive compensation is more likely to provide a less accurate result than that obtained through actual measurement.
The necessity for compensating for generic electrode wear is also recognized in the Inoue '460 patent, earlier cited. In that patent Inoue suggests inclusion of a gap length monitoring circuit. When the gap exceeds a predetermined length, though insufficient in length to extinguish continued sparking, the monitoring circuit signals the numerical control apparatus, and the latter, in response lowers the drive assembly, carrying the electrode forward, a small amount to place the electrode's tip closer to the workpiece. As an advantage the present invention does not require either the direct measurement of electrode to workpiece as suggested in Inoue '460 or the predictive technique to compensate for electrode wear, but instead insures that the electrode remain of the proper length, despite erosion.
Moreover, with either type of prior art gap length correction technique, a point is eventually reached during machining where the electrode is completely worn down. The EDM apparatus must be shut down and the worn tool replaced, a procedure required all too frequently. Electrode replacement delays machining, and takes the attendant away from other duties. Those skilled in the art appreciate that the efficiency of EDM processing should be improved considerably if the necessity for frequent electrode replacement was eliminated altogether. As a still further advantage, the present invention eliminates such replacements, permitting virtually unattended continuous EDM operation.
Direct or indirect measurement of electrode to workpiece distance in those prior EDM, such as found in the cited Inoue and Shimizu patents, which provide such kind of spark gap control, encourages the cutting tool to follow any surface undulations in the workpiece, disadvantageously repeating those undulations in the cut surface. The effect is to perpetuate a rough surface, instead of obtaining an absolutely smooth surface. As a further advantage the present invention is unaffected by the original condition of the workpiece. Hence, with the present invention a smooth surface may be cut irrespective of surface roughness.
To remove large volumes of material, the sinking EDM process has necessarily been used, despite its difficulty and slowness, and despite the fact that the fabrication of the three dimensional shaped electrodes is expensive and requires considerable lead time. As an advantage another inventive aspect of the present invention permits removal of large volumes of metal without employing sinking or custom shaped electrodes. Advantageously, the present invention also permits chunks of metal to be sequently cut from a workpiece.
Accordingly, a principal object of my invention is to permit three dimensional numerical controlled electric discharge machining apparatus to perform electro-machining virtually unattended.
A further object of my invention is to increase the efficiency of electric discharge machining processes by minimizing the necessity for replacing electrodes due to electrode erosion, minimizing EDM "down time" and minimizing demands for the attendant's attention.
A still further object of my invention is to provide electric discharge machining apparatus with the capability of reserving and storing portions of the cutting electrode and to replenish spent portions of the cutting electrode from the reserved portion.
A related object of my invention is to replenish worn portions of an EDM electrode automatically, essentially without interruption of the electric discharge machining process.
An additional object of my invention is to enable EDM apparatus to form a cavity by "scooping" out chunks of metal from the workpiece, requiring minimal cutting, whereby the desired cavity may be quickly roughed out to approximate dimensions, and then to finish the cavity with great precision, even to engrave the cavity sides with artwork or texture, currently unobtainable without special EDM setups.
A still additional object of my invention is to perform both roughing and finishing operations without using custom made electrodes in a sinking operation, and to perform the foregoing operations under software control, leaving the machining apparatus essentially unattended.
A more specific object of the invention is to provide a new form of wire electrode that employs the tip end of wire for generation of EDM sparks, whereby the wire's tip may be applied as a point source of sparks to three dimensional shaping of metal surfaces.
An ancillary object of the invention is to minimize wire electrode deflection during machining arising from electromagnetic influences or other external forces.
Another more specific object of the invention is to provide a new form of EDM electrode and accompanying means for moving that electrode to produce a scooping action during electric discharge machining, whereby metal chunks are scooped from a workpiece.
And a still additional object of the invention is to enable direct monitoring and maintaining of the spatial position of the sparks occurring in the EDM gap during the electro-erosion process, eliminating effects of electrode wear and the need to monitor EDM gap length.